Many accelerators consist of a means of applying high voltage to a large metal box called a terminal, an ion source for generating ions which are to be accelerated, and an acceleration tube in which the ions traverse the high voltage and thereby gain energy. In order to obtain voltages in the megavolt range in a room of reasonable size, the terminal is confined inside a pressure tank which is filled with insulating gas at high pressure, the purpose of the gas being to prevent sparks across the short distance from terminal to tank. Also since the acceleration of ions would be impeded if they ran into gas molecules, the inside of the accelerator tube is evacuated.
Most accelerators use a plasma ion source consisting of a glass bottle inside which a gas discharge is maintained by power coupled into the discharge from the coil of a radiofrequency oscillator. Ions leaving the surface of the plasma in this discharge are accelerated and focused by a DC potential between the plasma and an exit canal, which is typically a small hole in an aluminum rod.
In the prior art, sources of the field ionization type have been installed in accelerators. Such a source typically consists of a very fine needle with a hemispheric point having a radius on the order of 500 Angstroms, maintained at a positive voltage of the order of 10 to 20 kilovolts in a gas at a pressure less than 10.sup.-4 millimeters of mercury. The gas near the tip is ionized by the extremely high electric fields at that point and accelerated along paths appearing to radiate from a small region near the center of the hemisphere. This property of emanation from an apparent source of size as small as 10 Angstroms result in extremely high brightnesses, as great as 10.sup.8 amperes per steradian per cm.sup.2 of source size at 10-20 KV.
Field emission sources of electrons are also well known in the prior art of scanning electron microscopy. These consist of a tip of similar or identical geometry to that of a field ionization source, operated at negative potential and in high volume.
Prior art teaches how an accelerator beam may be directed through a magnetic or electrostatic energy analyzer, 54 and how a signal can be derived from a pair of slits 55 at the focus of an analyzer, the signal from one slit increasing and the signal from the other decreasing if the center of the beam approaches the first slit because of a deviation from the desired energy. This procedure has the advantage of eliminating a temperature-dependent, discharge-prone voltage-divider network for measuring the high voltage, and of substituting an analyzer which may be made thermally, mechanically and vibrationally stable and in which deflecting voltages or magnet currents may be controlled to 1 part in 10.sup.6. Stabilities have been achieved by means of this principle which are as great as 200 Volts in 4 megavolts in ion accelerators using radiofrequency sources and 10 volts in 1 megavolt in electron accelerators using thermionic filament electron sources. The limiting factor is the size of the accelerator beam at the analyzer focus. In the electron accelerator this size has been reduced to 40 microns in an analyzer with radius 0.5 meters. Assuming control of the center of the spot position to 2.5 microns, the momentum uncertainty in the analyzer is 0.5.times.10.sup.-5 and the resulting energy uncertainty is 1 part in 10.sup.5. In ion accelerators having radiofrequency ion sources with an exit canal of 0.5 mm diameter, this spot size is typically a fraction of a millimeter and the accompanying energy uncertainty is 1 part in 10.sup.3 to 10.sup.4.
The width of the particle beam at the analyzer focus is fundamentally determined by the geometry of the analyzer and the emittance of the particle beam. The emittance of a particle beam is proportional to the area which it occupies in the phase space composed of its transverse position and momentum. According to Liouville's theorem of mechanics, this area does not change as the beam passes through a linear optical system. Thus when the beam from a given ion source arrives at an analyzer, where its maximum transverse momentum is defined by the instrument aperture and the distance of this aperture from the exit slits, there will be a minimum size smaller than which the beam cannot be focused without loss of current.